This is only relevant to external APIs for use of the special gmt read|write modules: It should be possible to append more resources to an existing dataset (for those data types that allow this). E.g., in MATLAB:

The mex part simply needs to realize that the l.h.s. pointer exists already and that uppercase -TD was given (which implies append). The external API should check that the operation is allowed (the GMT API will complain if no checking is done).